During exploration and reservoir assessment and development in the oil and gas industry, hydrocarbons, such as oil and gas, as well as geological structures that tend to bear hydrocarbon, may be detected based on their properties (e.g., mechanical and electromagnetic (EM) properties) that are different from those of the background geological formations.
Electromagnetic (EM) measurements are commonly used in oil and gas exploration. Among the EM properties, the resistivity (ρ), which is an inverse of the electrical conductivity (σ), is particularly useful. This is because hydrocarbon-bearing bodies, such as oil-bearing reservoirs, formations containing methane hydrate, and gas injection zones, have higher resistivities compared with their background geological formations. For example, hydrocarbon-bearing reservoirs typically have resistivities one to two orders of magnitude higher than those of the surrounding shale and water-bearing zones. Therefore, resistivity mapping or imaging may be useful in locating zones of interest in contrast to the background resistivity. This method has been used successfully in both land and seafloor exploration.
Resistivity mapping may be achieved by generating an EM signal above the formations of interest and receiving the resulting EM field at selected locations. The received data is affected by a number of parameters, for example, the distance between the EM signal source and the receivers. EM field frequency, polarity of the EM waves, depth and thickness of the reservoir, resistivity of the hydrocarbon bearing zones, and the surrounding geological formations. In marine applications, the received signal may depend on the resistivity of the seawater, which may be a dynamic variable that depends on the water temperature, salt content, etc.
The EM signal may be from natural sources or from artificial sources. Among the EM methods, magneto-telluric (MT) methods rely on the naturally-occurring EM fields in the stratosphere surrounding the earth. Because carbonates, volcanics, and salt all have large electrical resistivity as compared with typical sedimentary rocks, MT measurements may produce high-contrast images of such geostructures. MT measurements are particularly useful in examining large-scale basin features and for characterizing reservoirs below basalt (volcanics) layers beneath a sea bed.
Controlled source electromagnetic (“CSEM”) methods use EM transmitters, called sources, as energy sources, and the receivers measure the responses of the geological structures to the transmitted signals. The transmitter may be a direct current (DC) source, which injects a DC current into the geological formations. DC currents are typically injected into the formations using contacting electrodes. Recent EM measurement methods use EM sources that produce time-varying electrical and/or magnetic (EM) fields. The EM fields may be a pulse generated by turning on and off an EM transmitter, and in this case, the receivers effectively measure a pulse response of the geological structures. EM measurements may use a transmitter that transmits a fixed frequency or a range of frequencies. The higher frequency EM sources permits resolution of finer structures, whereas the lower frequency EM sources allows one to probe deeper into the formations.
In marine explorations, low-frequency EM methods are typically used. The low-frequency EM waves may induce a current, i.e., the Faraday (eddy) current, to flow in the earth formation and in the sea water. The current density depends on the impedance of the earth formation and the sea water. A voltage drop across two locations produced by the current may be measured and used to infer the resistivity distribution in the formation. Alternatively, one may measure the secondary magnetic fields produced by the induced current.
As discussed, CSEM uses an artificial EM source to generate controlled EM fields that penetrate the ocean and the subsea formations. As illustrated in FIG. 1, in a conventional CSEM method, an electrical dipole transmitter 11 is towed by a ship 10 at a short distance above the seabed 12. The transmitter 11 induces EM fields in the sea water 14, geological layers 15 and 16 and the oil-bearing layer 17.
To detect the EM signals, a number of receivers 13 are deployed on the seabed 12. The EM signals measured by the seafloor receivers 13 may then be used to solve an inverse problem to estimate the resistivity distributions in the geological structures, including layers 15, 16, and 17. When the transmitter 11 is not used, the receivers 13 may be used to detect EM signals induced by the naturally-occurring MT fields.
A typical structure of a traditional receiver 20 is illustrated in FIG. 2. As shown, the receiver 20 typically has a body (frame) 1 with arms 3a, 3b, and 3c attached thereto. At the end of each arm 3a, 3b, or 3c is an electrode 5a, 5b, or 5c, as well as an electrode 5d located near the receiver frame 1, which are for detecting the electrical field signals. In addition, a receiver typically includes two additional horizontal electrodes (not shown) that are orthogonal to electrodes 5a and 5b. Typically the receiver structure can accommodate magnetic field sensors as well. These may or may not be integrated into the arms 3a, 3b or 3c. The receiver frame 1 encloses the receiver circuitry 2. The circuitry 2 connects, through electrical cables 4a, 4b, 4c, and 4d, to electrodes 5a, 5b, 5c, and 5d. The cables 4a, 4b, 4c, and 4d may be enclosed in protecting enclosures forming the “arms” 3a, 3b, and 3c. The arms are typically made of insulating materials, such as plastic. The electrodes 5a, 5b, 5c, and 5d are typically made of sandwiched Ag—AgCl. Because these electrodes need to be in direct contact with sea water, they need constant maintenance to prevent problems arising from corrosion.
The electrodes are used in pairs to measure different components of the electrical fields. The electric field is traditionally measured as the voltage drop V between two opposing electrodes. For example, the transverse component of the electric field is measured by the dipole configuration formed by electrodes 5a and 5b in the horizontal direction, and the vertical component of the electric field is measured by the dipole configuration formed by electrodes 5c and 5d in the vertical direction. Although not depicted, it is also common to measure the electric field in a direction that is perpendicular to the electrodes 5a-5b and the electrodes 5c-5d. Such a direction would be into the page in FIG. 2. Further, it is also common to include magnetic field sensors to measure the magnetic field intensity, typically in orthogonal directions.
For the same electric field E, the detected voltage V would be larger if the distance d between the opposing electrodes is larger because V=d E. However, it is impractical to increase d beyond a certain limit for the purpose of increasing the sensitivity of the measurements. This is because it will be more difficult to transport and deploy large-sized arms, and the reliability of the receivers also suffers. On the other hand, the arms cannot be too short because the receiver needs to have sufficient sensitivity for EM fields, especially in the low frequency regime, e.g., between 0.02 Hz and 10 Hz. The lengths of the arms 3a and 3b supporting the electrodes 5a and 5b in a typical receiver are around 4 meters, and the vertical arm 3c is typically extended about 2 meters from the frame 1.
Due to the large size of the receivers, these electrode arms and the receiver body (frame) are usually transported as separate components and assembled before deployment. The assembling of the receivers is a very time consuming process during an operation. In addition, frequent connecting and disconnecting the electrodes, arms and cables from the receiver circuitry may lead to reliability and sensitivity problems. Recent designs of these type of receivers have attempted to partially address the problem by designing flexible arms which may be folded during storage or transport but which are unfolded prior to deployment for sea bottom measurements. Finally, in order to measure the low frequency EM fields, the electrodes are required to be non-polarizable, typically Ag—AgCl electrodes. This presents a technical challenge to make sensitive receivers.
The receiver sensitivity determines the sensitivity of an EM survey. The resolution of the survey image may also be affected by the receiver sensitivity. A major source of noises in the receiver 20 is the bandwidth limited Johnson noises of the receiver circuitry 2. State-of-the-art receivers can achieve a noise level of 1 nV/√{square root over (Hz)} or better at the input stage to the receiver circuitry 2. If all other sources of noise are properly managed, the total noise level of the receiver 20 may be controlled to a level of around 100 pV/ml√{square root over (Hz)} for the transverse electric field component and around 300 pV/m/√{square root over (Hz)} for the vertical electric field component. This sensitivity determines the limit of how sensitive the EM survey will be to deeply buried structures within the subsurface and in general the resolution and fidelity of any image derived therefrom.
Improvement of the receiver sensitivities is limited by the impedance of the input stage of the receiver circuitry 2 and by the noise generated in the antenna electrodes 5a, 5b, 5c, and 5d. In addition, the long arms 3a, 3b, and 3c supporting the electrodes 5a, 5b, 5c are subject to vibrations induced by sea currents, and may even resonate acoustically. Such vibration or resonance significantly increases the noise level.
Due to the technical difficulties in measuring the electric fields by voltage drops, it may be more advantageous to measure an electric field E by measuring electric current densities J and the electric conductivity σ of the sea water. The electric field E may then be derived using the Ohm's law,E=J/σ,  (1)where J is the current density, and σ is the electric conductivity. This principle has been applied to measuring electric fields using opposing conductive plates in a cubic or rectangular receiver frame, as taught in French Patent 8419577, issued to Jean Mosnier, and in WO 2006/026361 by Steven Constable. This French Patent and the WO 2006/026361 are incorporated by reference in their entireties. One example of such a receiver is illustrated in FIG. 3.
As shown in FIG. 3, a receiver 300 includes conductive plate electrodes 31a, 31b, 31c, and 31d attached to the outside surfaces of the cubic receiver frame 30. These conductive plates are insulated on the sides 37a, 37b, 37c, and 37d facing away from the sea water. Electrodes 31a and 31b are connected to a coupling device 34b via cables 32a and 32b, respectively. Similarly, electrodes 31c and 31d are connected to a coupling device 34a via cables 32c and 32d, respectively. An electric current in the horizontal direction will flow into electrode 31a, through cable 32a and coupling device 34b, and then out of electrode 31b back to the water. The current density in the horizontal direction defined by electrodes 31a and 31b can then be measured by the measurement circuitry. Similarly, an electric current density in the vertical direction defined by electrodes 31c and 31d may also be measured by the measurement circuitry. In addition, an electric current density in the third orthogonal direction may be measured by using a third pair of electrodes (not shown) outside the receiver box 30. The coupling devices 34a and 34b couple the current flows to the electronic circuitry 36. The electronic circuitry 36 amplifies the currents and feed the signals to the rest of the measurement circuitry.
While these prior art receivers have been useful in oil and gas exploration there remains a need for better receivers that are easy to use and can provide robust measurements.